The variabe for the "getting-started" manual goes away and is
replaced by this one for the new "overview-manual."
(From yocto-docs rev: 45fc9beac6db4c40c3660fc9e54cc11e9c1f96c4)
Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>